Optical sensing devices can be utilized to detect small concentrations of chemicals or biological materials, and thus have many applications in the areas of pollution control, defense, corrosion detection, explosives detection, water quality monitoring, biological sensing and quality control in manufacturing. There have been a number of attempts to create a sensing device based on optical fiber technology employing the principles of absorption and fluorescence spectroscopy. As would be appreciated by those skilled in the art, the sensitivity of such an optical fiber based device is primarily related to the power fraction of the guided mode field that is available to be overlapped with the material to be sensed.
Depending on the structure or geometrical configuration of the fiber based sensor, the fraction of the guided light that is available for material interactions can vary widely. A number of attempts at developing a fiber based sensor have made use of the evanescent tails of the modal field (evanescent sensing) or alternatively the central portion of a mode guided within an air core. Some examples of fiber-based sensors that utilize evanescent based sensing principles include tapered fibers, D-shaped fibers, microstructured optical fibers (MOFs), photonic crystal fibers (PCF) and even nanowires where almost all the mode becomes available for sensing. Other examples of waveguide geometries that can provide access to light-material interactions for sensing that do not necessarily rely on making use of the evanescent field include capillary tubes and hollow core photonic bandgap fibers.
However, each of these categories of fiber-based sensors described above has a number of disadvantages. For non-evanescent sensors, while there is often excellent light matter overlap, there are a number of technical difficulties in implementing a practical sensor. For geometries in which the optical field available for light-matter interactions is located within an air core (such as capillary tubes or hollow core MOF), the requirement for a cladding region to surround this core and thus provide confinement for the mode restricts options for sensing the local environment. A further drawback of these devices is that it is generally necessary to load sensors of the type at their ends.
In contrast, nanowires provide ready external access to the optical field. However, the most significant practical restriction with these devices is related to difficulties in the handling of nanowires due to their small dimensions and the resultant issues with integration, fragility and contamination. While the well-studied case of D-shaped fibers offers a more practical alternative, evanescent field based sensors of this type can offer only relatively small light-matter overlap, and hence low efficiency. A secondary and related disadvantage is that these fibers do not efficiently capture fluorescent photons so that they may be propagated as a fluorescent signal to a location where the signal may be analyzed.
One example of an evanescent type fiber based sensor that attempts to address the capture and propagation of fluorescent photons is illustrated in FIG. 1 which is a MOF sensor 100 that includes a central core region 110 for propagation of light having an excitation wavelength 130 surrounded by three longitudinally extending elongate channels or chambers 120, 121, 122. In this example, channel 121 is filled with a fluorescent material 140 within solution which on excitation by a portion of light of excitation wavelength 130 arising from the evanescent field of light propagating along central core region 110 will cause fluorescence 145 of light of fluorescent wavelength 131 which radiates substantially uniformly in all directions. A component of this fluorescence 145 is then captured by central core region 110 and then propagated along MOF sensor 100 for detection either by reflection at a proximal end 150 or by transmission at a distal end 160 of MOF sensor 100.
Whilst a MOF sensor of this type has many convenient features such as ease of filling, close to real time measurements and deployment as they may be readily connected to conventional fiber based systems, in practice the relatively low light matter overlap and the associated low capture efficiency of the fluorescent photons in central core region 110 has meant that MOF sensors of this configuration have only been able to demonstrate relatively low sensitivities.
Accordingly, there is a need for an optical fiber based sensor utilizing fluorescence that is capable of providing increased sensitivity that is readily deployable.